Boatbuilding Blog

January 22, 2008

Outboard Mockup

Filed under: Building - After Flipping, Hardware — tomlarkin @ 1:43 pm

Here are some shots of an outboard motor mockup I made to test the fit of the real motor. It’s a full-scale copy of the 45 HP Honda on my runabout. First, I took pictures of the real motor with a square for scale, then traced them full-sized on paper using a projector in a conference room at work.

I transferred the drawing to some old foundation insulation by poking holes through the drawing with a nail, then cut out the foam with a keyhole saw. I did this a number of times to build up the thickness, and glued it all together with construction adhesive.

That's the van in the background I knew I held on to all that stuff for a reason!



I hammered in some steel rod scrap for strength. Then I formed it to shape with a Surform and covered it in epoxy. I made a motor mount with about the same dimensions as the real one and welded the parts together to make a mount that lifts and swivels about the same as the real one. I’m looking forward to seeing it mounted in the boat! Should be somewhat easier to play with than a 300 pound real motor.

1-to-1 scale model  Epoxy and wood flour coating  Tilt-n-swivel mount made from scrap and dowels

I now know that a 20-inch motor means that the transom height is 20 inches, measured from the top of the mount vertically down to the bottom of the boat. The motor  will be in the water deeper or shallower depending on the distance of the waterline to the bottom of the boat. The idea is the anti-cavitation plate should line up with the bottom of the boat to ensure smooth water flow from the hull to the propeller. This is easy to lay out on my boat because the motor well mount is vertical.

My motor plans for now are to buy a new Yamaha High Thrust 50 HP motor. The High Thrust motors are designed to move displacement boats at non-planing speeds. They have oversized gearboxes made to run slower than most motors. The propellers are somewhat larger too, helping with slow-speed acceleration and reverse.  They’re supposed to be quiet and low-vibration too, which I think is very important. This boat only really needs about 20 HP to push it at hull speed, but I want extra power to drive it safely out of the most adverse conditions. I imagine going through Deception Pass against the tide and a strong wind pushing up the waves.  I’m still looking at the equivalent Mercury too – called a BigFoot. They’re both 4-stroke EFI engines with auto-tilt and trim. They cost about $6000 new – we’re going to sell the van and runabout to offset some of the cost. Anyone in the market for a nice van with a queen-size bed or a 16 foot runabout with a Honda? Runs good!



  1. We talked about what kind of plans you might have for that styrofoam model of the outboard motor after you are done with it. I wish you had put in a bigger picture. The model is almost a perfect likeness in shape and the mount that you made copies the tilt and lift so that space is easier to judge. This detail is important enough to be worth all that extra effort!

    Comment by Meryll — January 24, 2008 @ 12:56 am

  2. Tito
    I thought you were crazy, then I saw what you were doing. Clever boy!

    Comment by Joe — March 1, 2008 @ 6:27 am

  3. […] am Three years ago when I was designing the outboard motor area I used my Honda 45 HP as a model. I made a full-size copy of the motor out of foam and designed the space around that. Since then I decided to use a larger motor – the […]

    Pingback by Motor Mounting « Boatbuilding Blog — April 29, 2010 @ 10:38 am

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